1. Field of Invention
The present invention relates to methods and compositions for use in pressed powder metallurgy.
2. Description of Related Art
In pressed powder metallurgy, a substantially dry metal powder composition is charged into a die cavity of a die press and compressed to form a green compact. Pressing causes the metal powder particles in the metal powder composition to mechanically interlock and form cold-weld bonds that are strong enough to allow the green compact to be handled and further processed. After pressing, the green compact is removed from the die cavity and sintered at a temperature that is below the melting point of the major metallic constituent of the metal powder composition, but sufficiently high enough to strengthen the bond between the metal powder particles, principally through solid-state diffusion. Some metal powder compositions include minor amounts of other metals and/or alloying elements that melt during sintering to facilitate liquid phase sintering of the non-melting major constituent of the metal powder composition. This increases the bonding strength between the major metallic constituent of the metal powder composition and typically increases the final density of the sintered part.
In most pressed powder metallurgy applications, it is necessary to add a lubricant to the dry metal powder composition before it is pressed to form the green compact. The most commonly used lubricants in pressed powder metallurgy are ethylene bis-stearamide wax and zinc stearate, but other lubricants are also sometimes used. Lubricants help the individual metal powder particles flow into all portions of the die cavity, allow for some particle-to-particle realignment during pressing and can serve as release agents that facilitate removal of the green compact from the die cavity after pressing. The least amount of lubricant necessary to obtain good flow and release is used.
Conventionally, the lubricant is removed from the green compact by gradually heating the green compact at a relatively low heating rate (e.g., ˜15° F./min) until the lubricant melts, boils and/or decomposes and is completely removed from the pressed part. This “delubing” is typically accomplished during an initial heating or preheating stage at the beginning of the sintering process. This can be accomplished in a batch furnace or in a continuous furnace. In a continuous furnace, the green compact is placed on a conveyor that moves the part slowly into and through a sintering oven. The slow movement of the conveyor allows the temperature of the green compact to increase at a slow rate, allowing the lubricant to melt and then boil and then gas off. Most of the remaining lubricant residue is decomposed and burned out as the temperature of the green compact increases. Some small quantity of the lubricant may diffuse into the base metal and contribute carbon to the final part. The lubricant is completely removed from the green compact at a temperature that is substantially lower than the final sintering temperature. In a batch furnace, the temperature is gradually increased to remove the lubricant prior to sintering that may be programmed to run at different conditions.
To maximize the opportunity for the individual metal particles to bond to each other, it has long been the practice to sinter the green compact at a peak sintering temperature for a significant amount of time, typically on the order of 30 minutes or more. Allowing the part to soak or dwell at the peak sintering temperature for this period of time is believed to increase the likelihood that individual metal particles will bond via solid-state diffusion. The slow movement of the conveyor or the temperature profile in a batch furnace insures that the green compact receives a lengthy soak or dwell time in the hot zone of the sintering oven.
Ideally, the sintered density of a final part would be 100% of the theoretical density of the metallic constituents of the metal powder composition used to form the part. However, the sintered density of parts formed from most conventional metal powder compositions does not approach 100% of theoretical density. Using conventional high carbon or low alloy steel metal powder compositions and pressed powder metallurgy methods, only a sintered density of about 93% to 94% of theoretical density can be achieved in one pressing and sintering. For stainless steels, sintered densities are typically less than 90% of theoretical density for conventional powder metallurgy compositions. Additional processing steps, such as forging and repressing are required to increase the density of the sintered metal part.